A brand new examine means that pulsars will not be the supply of an surprising surplus of antimatter particles detected by a space-based experiment. Darkish matter stays a viable different rationalization.
The Earth is consistently being bombarded by cosmic rays — excessive vitality protons and atomic nuclei that pace via area at almost the pace of sunshine. The place do these energetic particles come from? A brand new examine examines whether or not pulsars are the supply of 1 explicit cosmic-ray conundrum.
An Extra of Positrons
In 2008, our efforts to know the origin of cosmic rays hit a snag: information from a detector referred to as PAMELA confirmed that extra high-energy positrons had been reaching Earth in cosmic rays than concept predicted.
Positrons — the antimatter counterpart to electrons — are considered primarily produced by high-energy protons scattering off of particles inside our galaxy. These interactions ought to produce lowering numbers of positrons at larger energies — but the info from PAMELA and different experiments present that positron numbers as a substitute go up with growing vitality.
One thing have to be producing these further high-energy positrons — however what?
Clues from Gamma-rays
One of many main theories is that the surplus positrons are produced by close by pulsars — quickly rotating, magnetized neutron stars. We all know that pulsars steadily spin slower and slower over time, dropping energy as they spew a stream of high-energy electrons and positrons into the encompassing interstellar medium. If the pulsar is shut sufficient to us, positrons produced in and round pulsars may make it to Earth earlier than dropping vitality to interactions as they journey.
May close by pulsars produce sufficient positrons — and will they diffuse out from the pulsars effectively sufficient — to account for the high-energy extra we observe right here at Earth? A staff of scientists now addresses these questions in a brand new publication led by Shao-Qiang Xi (Nanjing College and Chinese language Academy of Sciences).
To check whether or not pulsars are accountable for the positrons we see, Xi and collaborators argue that we must always search for GeV emission round candidate sources. Because the pulsar-produced positrons diffuse outward, they need to scatter off of infrared and optical background photons within the surrounding area. This may create a nebula of high-energy emission across the pulsars that glows at 10–500 GeV — detectable by observatories just like the Fermi Gamma-ray Space Telescope.
Two Pulsars Get an Alibi
Xi and collaborators fastidiously analyze 10 years of Fermi LAT observations for 2 close by pulsars which have been recognized as doubtless candidates for the positron extra: Geminga and PSR B0656+14, positioned roughly 800 and 900 light-years away from us.
The consequence? They discover no proof of prolonged GeV emission round these sources. The authors’ higher limits on emission from Geminga and PSR B0656+14 give these objects an alibi, suggesting that pulsars can doubtless account for less than a small fraction of the positron extra we observe.
So the place does this go away us? If pulsars are cleared, we might want to look to different candidate sources of high-energy positrons: both different close by cosmic accelerators like supernova remnants, or extra unique explanations, just like the annihilation or decay of high-energy darkish matter.
“GeV Observations of the Prolonged Pulsar Wind Nebulae Constrain the Pulsar Interpretations of the Cosmic-Ray Positron Extra,” Shao-Qiang Xi et al 2019 ApJ 878 104. doi:10.3847/1538-4357/ab20c9
This publish initially appeared on AAS Nova, which options analysis highlights from the journals of the American Astronomical Society.